1. Field of the Invention
The invention relates to control of transport refrigeration systems of the type that have at least heating and cooling capabilities and dual compressor speed capability.
2. Description of the Prior Art
One typical transport refrigeration system such as is used to cool and at times heat a trailer or the like is controlled thermostatically by what is considered in the art to be a four stage controller. If the system has the capability of dual speeds along with the capability of either heating or cooling, the normal mode of four stages of operation with successively descending temperatures in the trailer would be as follows: high speed full cool, low speed full cool to the setpoint temperature, low speed full heat, and high speed full heat. With ascending temperatures from well below the setpoint to well above the setpoint temperature, the reverse order of stages of operation prevails.
With some systems, capacity reduction is also available, and then the successive stages of operation with sensed temperatures from well above to well below the setpoint temperature are: high speed full cool, low speed part cool, low speed part heat, and high speed full heat. Again, the reverse order of stages of operations prevails with ascending temperatures.
It is to be noted that in the first arrangement low speed part cool and low speed part heat are not available, while in the second arrangement low speed full cool and low speed full heat are not available.
Systems as above described have been commercially available for a number of years from the assignee of this application. They use an electronic thermostat basically comprising three temperature responsive switch means controlling a pair of relays, one of which has been characterized as a speed relay and the other as a heat-cool relay. Such a device in electronic form is available from the assignee under the trademark "Thermoguard".
It would be relatively desirable, in my estimation, if additional stages of control were not only available, but were also able to be obtained using a tried and proven thermostat such as noted above and with only a reasonable increase in cost in obtaining the additional stages of operation. It is the aim of this invention to provide a method and system in which these desirable results are obtained as well as other benefits derived from the arrangement of the invention.